Apparatus and method for removing sulfur from a hydrocarbon fuel

ABSTRACT

Method and apparatus for removing sulfur from a hydrocarbon fuel. The apparatus includes a combustion reactor ( 3 ) and a sulfur trap ( 4 ) and the combustion reactor ( 3 ) is adapted to operate with an air-to-fuel ratio below 1 and in the presence of steam. The sulfur trap ( 4 ) is located downstream the combustion reactor ( 3 ) and is adapted to remove sulfur compounds formed in the combustion reactor ( 3 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation patent application ofInternational Application No. PCT/SE2005/000490 filed 1 Apr. 2005 whichis published in English pursuant to Article 21(2) of the PatentCooperation Treaty and which claims priority to Swedish Application No.0400904-9 filed 2 Apr. 2004. Said applications are expresslyincorporated herein by reference in their entireties.

FIELD

The invention generally relates to an apparatus and method for removingsulfur from a hydrocarbon fuel. In particular, the invention relates toa system for catalytic treatment of a hydrocarbon fuel.

BACKGROUND

Hydrocarbon fuels, such as diesel, gasoline and natural gas, havegenerally a sulfur content that in most cases needs to be reduced ofenvironmental reasons and/or because sulfur is a potent poison forcatalysts and catalytic processes.

Sulfur is present in hydrocarbon fuels in the form of a variety ofsulfur compounds. The sulfur can be removed from the fuel in anindustrial hydrodesulfurization process (HDS) before the hydrocarbonfuel comes into actual use, such as in combustion or reformation. HDSmay be suitable for large-scale industrial processes where largequantities of hydrogen are available, but it is a costly and complicatedprocess and therefore not suitable in other applications, such asautomotive and fuel cell applications.

Another method is to remove sulfur after a fuel reforming step, such assteam reforming or partial oxidation, in which step the fuel iscatalytically reformed into smaller hydrocarbons and hydrogen, and inwhich the sulfur is converted into H₂S. This method is relativelyeffective and useful in many situations. H₂S can readily be removed froma gaseous stream by passing the sulfur-containing gas over a materialthat can react with the sulfur, such as ZnO, and thereby purge the gasstream of sulfur. However, this method has the disadvantage that thecatalyst in the reactor will suffer from poisoning which results inshort lifetimes for the catalytic system.

U.S. 2003/0188475 describes an example of a fuel reforming system wherethe sulfur trap has been incorporated after the catalytic reformer. Inthe disclosed system the fuel is initially vaporized and thencatalytically converted in an autothermal reformer into a hydrogen richgas before passed over the sulfur trap. The product is then fed via awater gas shift reactor and a catalytic preferential oxidation reactorto a fuel cell.

To avoid or at least reduce the problems related to sulfur contaminationof catalysts, focus has generally been set on developing catalysts thatare more resistant to sulfur or catalysts that are less expensive sothat each replacement of poisoned catalyst becomes less costly.

SUMMARY

One object of the present invention is to provide an apparatus and amethod for removing sulfur from a hydrocarbon fuel that eliminates or atleast reduces the problems related to sulfur contamination of catalystsin a system for catalytic treatment of a hydrocarbon fuel.

The invention concerns an apparatus for removing sulfur from ahydrocarbon fuel, and the invention is characterized in that theapparatus comprises a combustion reactor and a sulfur trap. Thecombustion reactor is adapted to operate with an air-to-fuel ratio below1.0 and in the presence of steam. The sulfur trap is located downstreamof the combustion reactor and is adapted to remove sulfur compoundsformed in the combustion reactor. Such a combustion reactor operates inthe absence of catalysts and converts parts of the fuel into smallercomponents as well as converts the fuel content of sulfur compounds intoeasily removeable compounds such as H₂S. By locating the sulfur trapbetween the combustion reactor and a subsequent catalytic reactor, thesulfur can be removed from a catalytic system before the catalysts inthe system have come into contact with the sulfur. The present inventionthereby eliminates, or at least minimizes sulfur poisoning of thecatalysts in the catalytic reactor and thereby increases the life timeof the catalytic part of the system.

The invention also concerns a method for removing sulfur from ahydrocarbon fuel that is characterized in that the fuel in a first stepis fed to a combustion reactor that operates with an air-to-fuel ratiobelow 1.0 and in the presence of steam. In a second step the fuel is fedto a sulfur trap located downstream of the combustion reactor and thesulfur trap is adapted to remove sulfur compounds formed in thecombustion reactor. This method makes it possible to remove the sulfurfrom the fuel before the fuel comes into contact with any catalysts thatwould be present in a catalytic reactor in a subsequent step of a methodfor catalytic treatment of a hydrocarbon fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to thefollowing drawings in which FIG. 1 schematically illustrates oneadvantageous embodiment of the invention.

DETAILED DESCRIPTION

In this context, hydrocarbon fuel relates to any hydrocarbon fluidsuitable for being used as a fuel, such as diesel, gasoline, ethanol,methanol, di-methyl ether and aviation fuels.

FIG. 1 schematically shows an advantageous embodiment of the invention.Hydrocarbon fuel 1 and steam/air 2 is fed to a combustion reactor 3 inwhich the hydrocarbon fuel is combusted under fuel rich conditions;i.e., the air-to-fuel ratio is below 1, in the presence of steam. Inthis combustion reaction the hydrocarbon fuel is partially broken downinto smaller molecules and all or most of the sulfur is converted intoH₂S. A sulfur trap 4 is located downstream the combustion reactor 4. Bycontacting the outgoing flow from the combustion reactor 3 with thesulfur trap 4, the sulfur compounds, generally H₂S, are removed from thehydrocarbon fuel. A catalytic reactor 5, in which the fuel is furtherconverted catalytically, is located further downstream in the system.Typically, the catalytic reactor 5 could be a steam reformer or apartial oxidation reactor.

The combination of a combustion reactor 3 and a sulfur trap 4; i.e., theapparatus for removing sulfur from a hydrocarbon fuel according to theinvention, is located upstream of the catalytic reactor 5 thus providingan effective way of preventing catalysts in a catalytic system frombeing exposed to sulfur from the combusted hydrocarbon fuel.

It is noteworthy that the combustion reactor 3 converts the varioussulfur compounds into certain sulfur compounds, such as H₂S, that areeasily separated from the fuel. To achieve this, it is important thatthe fuel-to-steam and air-to-fuel ratios be adapted to the selected fuelso that the rich fuel combustion process is stable. In many situationsit is a principle aim to reform or convert the fuel to a large degree,and in these cases it is advantageous if the combustion reactor 3 notonly converts sulfur compounds, but also converts the hydrocarbon fuelinto smaller molecules as efficiently as possible. In such a situation,the combustion reactor 3 works as a pre-reformer. To enhance suchpre-reforming reactions, it is advantageous to mix the steam with theair before injecting the air/steam into the combustion reactor 3.

The combustion reactor 3 is optimally operated with air-to-fuel ratios(lamda) between around 0.2-0.5, depending on the hydrocarbon used, butother air-to-fuel ratios can also give a satisfactory result. Steam isrequired in this step because it is used in the conversion of the sulfurcompounds and it is also used for controlling the temperature in thereactor. Suitable temperature depends on such things as the type ofhydrocarbon fuel used, but a typical suitable temperature is 350° C. Thesteam-to-fuel ratio depends to a large degree on the type of fuel.

The operating temperature of catalytic fuel reforming reactors such assteam reformers and partial oxidation reactors, in which the fuel isclose to completely converted to small molecules, is much higher than inthe combustion reactor 3. Typical temperatures are in the approximaterange of 800-1200° C.

A suitable combustion process for the combustion reactor 3 is the socalled “cold flame combustion” process or the “cool blue flamecombustion” process, each of which are well known combustion reactionscenarios.

Most of the sulfur compounds formed in the combustion reactor 3 in thetype of combustion reaction described above will be H₂S. This compoundcan easily be separated by means of conventional sulfur traps such asthose containing ZnO. Another sulfur compound that may be formed in thecombustion reactor 3 is COS. The amounts of COS formed depends on theoperational conditions of the combustion reactor 3, but generally theamounts will be much smaller than the amounts of H₂S. By choosing asulfur trap with a suitable material, both H₂S and COS can besimultaneously removed. The sulfur trap is preferably a separate unitthat can be replaced after some time when the adsorption material hasbeen consumed to a certain prescribed degree. It should be noted thatFIG. 1 only gives a schematic view of the system. Naturally, the systemmay comprise further catalytic reactors and/or sulfur traps. Forinstance, in some cases it may be necessary to implement a furthersulfur trap downstream the catalytic reactor 5, as in a catalyticreforming system where some of the catalysts in subsequent catalyticreactors are ultra-sensitive to sulfur.

The invention is not limited to the above described embodiments, but anumber of modifications are possible within the frame of the patentclaims.

For instance, the catalytic reactor 5, or plurality of catalyticreactors located downstream of the sulfur trap 4 can be of various typesrelating to, for instance, the following catalytic processes:autothermal reforming, catalytic reforming, partial oxidation, steamreforming, exhaust gas catalytic oxidation, exhaust gas catalyticreduction, catalytic combustion, preferential oxidation and fuel cells.In these processes the fuel is further broken down and in many cases itis an advantage if the combustion reactor 3 works as a pre-reformer asmentioned above.

1. An apparatus for removing sulfur from a hydrocarbon fuel, saidapparatus comprising: a combustion reactor (3) and a sulfur trap (4),said combustion reactor (3) being adapted to operate such thatcombustion of the fuel is performed with an air-to-fuel ratio below 1and in the presence of steam, and wherein said sulfur trap (4) islocated downstream the combustion reactor (3) and is adapted to removesulfur compounds in the combustion reactor (3).
 2. The apparatus asrecited in claim 1, wherein the combustion reactor (3) is adapted tooperate with an air-to-fuel ratio between 0.2 and 0.5.
 3. The apparatusas recited in claim 1, wherein the combustion reactor (3) is adapted tobreak down at least long hydrocarbon molecules into smaller moleculesduring operation.
 4. The apparatus as recited in claim 1, wherein thecombustion reactor (3) is adapted to convert a large fraction of thefuel content of sulfur compounds into H₂S during operation.
 5. Theapparatus as recited in claim 1, wherein the combustion reactor (3) isadapted to operate at a temperature of approximately 300-400° C.
 6. Theapparatus as recited in claim 1, wherein the sulfur trap (4) containsZnO.
 7. A system for catalytic treatment of a hydrocarbon fuel, saidsystem comprising: a catalytic reactor (5) located downstream of acombustion reactor (3) and a sulfur trap (4), said combustion reactor(3) being adapted to operate such that combustion of the fuel isperformed with an air-to-fuel ratio below 1 and in the presence ofsteam, and wherein said sulfur trap (4) is located downstream thecombustion reactor (3) and is adapted to remove sulfur compounds in thecombustion reactor (3).
 8. The system as recited in claim 7, wherein thecatalytic reactor (5) is a fuel reforming reactor.
 9. The system asrecited in claim 7, wherein the catalytic reactor (5) is a fuelreforming reactor chosen from the group comprising; a steam reformer, anautothermal reformer and a partial oxidation reactor.
 10. The system asrecited in claim 7, wherein the system further comprises a plurality ofat least one of catalytic reactors and sulfur traps.
 11. A method forremoving sulfur from a hydrocarbon fuel, said method comprising: feedinghydrocarbon fuel to a combustion reactor (3) and in which combustion ofthe fuel is performed with an air-to-fuel ratio below 1 and in thepresence of steam; and feeding the fuel to a sulfur trap (4) locateddownstream of the combustion reactor (3), said sulfur trap (4) beingadapted to remove sulfur compounds formed in the combustion reactor (3).12. The method as recited in claim 11, wherein the combustion reactor(3) operates with an air-to-fuel ratio between 0.2 and 0.5.
 13. Themethod as recited in claim 11, wherein the combustion reactor (3) breaksdown at least long hydrocarbon molecules into smaller molecules.
 14. Themethod as recited in claim 11, wherein the combustion reactor (3)converts a large fraction of the fuel content of sulfur compounds intoH₂S.
 15. The method as recited in claim 11, wherein the combustionreactor (3) operates at a temperature of approximately 300-400° C. 16.The method as recited in claim 11, wherein the sulfur trap (4) is of atype containing ZnO.
 17. The method as recited in claim 11, wherein themethod further comprises utilizing a catalytic reactor (5) for removingsulfur from the hydrocarbon fuel, and wherein said combustion chamber(3) and sulfur trap (4) are located upstream the catalytic reactor (5).18. The method as recited in claim 17, wherein the catalytic reactor (5)operates as a fuel reforming reactor, such as a steam reformer or apartial oxidation reactor.
 19. The method as recited in claim 17,wherein the fuel is further treated in at least one of catalyticreactors and sulfur traps.